Instrumentation, air pollution abatement equipment, scientific instruments, medical devices such as needles and implants, fuel injection nozzles, spinnerettes and gas escapement orifices are some of the produces that require extremely small holes, including those holes with other-than-round shapes. Diesel fuel injectors require holes between 0.005 and 0.01 inches in diameter.
The traditional method of small hole drilling involves a drill bit that is subject to easy breakage, and a sensitive drill press in the hands of a skilled operator. The cost is usually high with broken drills and scrapped parts. If the workpiece is a hardened metal, the problems are compounded.
Electric discharge machining (EDM) has been employed from time to time in the drilling of small diameter holes. EDM has worked to a degree even though the results are usually far from optimum since most EDM equipment is designed to handle work on a much larger scale. Amperages, spark frequencies and overcuts that are ideal for machining a die segment, leave much to be desired in the task of drilling a fine, accurate hole with no appreciable layer of recast and solidified material on the hole surface.
EDM, however, still offers numerous advantages in the drilling of small diameter holes. One such advantage is that the hardness of the workpiece to be drilled is irrelevant as long as the material is electrically conductive and a spark can be forced to jump from an electrode to the workpiece. The rate of metal removal is a function of electrical conductivity and thermal characteristics of the workpiece. While extremely hard materials sometimes have a higher melting temperature, it is the melting temperature, not the hardness, that is the governing factor.
On the other hand, if the workpiece has no electric conductivity, the EDM process cannot be used. However, all metals and metallic alloys are electrically conductive to some degree and yield to the controlled cascade of sparks.
Another advantage of the EDM process is that, when properly controlled, the EDM hole drilling process is very accurate and has a high degree of stability. Because there is no direct contact between the electrode and the workpiece, there are no mechanical forces of the type found in conventional drilling. In small hole work, there is frequently not even a flow of dielectric fluid into the gap area to set up mechanical forces. The energy utilized in the actual metal removal process is divided into very high frequency sparks which are closely controlled with electronic systems now available.
With the EDM process there is no undue heat generation or any significant mechanical forces involved. Consequently, there is no part distortion. As a result, extremely thin and/or fragile parts can be successfully drilled with the EDM process.
Also, once the EDM job is set up, it can be completely cycled in an automatic fashion. As a result, skilled operators are not required.
The tool cost per hole is also extremely low. To drill holes under 0.015 inches in diameter, a tungsten alloy wire electrode which comes in spools is used. For sizes over 0.015 inches, straight rods are usually used. By way of example, typically hundreds of dollars of tungsten alloy wire will furnish enough electrode material to drill millions of holes in diesel fuel injector nozzles.
Still another advantage of the EDM process is the ability to vary the hole diameter within a limited range by simply changing current parameters without changing the electrode itself. By contrast, the diameter of mechanically drilled holes is determined by the diameter of the cutting tool. In order to resize the holes the size of the tool must be changed or a secondary operation must be performed. In EDM there is always an overcut comprising the spark gap between the electrode and the workpiece. The gap is a direct function of currrent flow and the frequency with which it is applied. In particular, the higher the current flow or the lower the frequency of sparking, the greater the gap. On normal EDM work, the gap may be anywhere from 0.001 to 0.003 inches on a side. Thus, in hole drilling work, the hole diameter is always larger than the tool itself. In small hole work the overcut is relatively small, but it can be closely controlled.
There are no burrs on the holes produced by the EDM process. Metal is eroded away in very minute globules to leave a non-directional type of surface finish. In the amperage and frequency ranges utilized in small hole EDM drilling, a recast surface is virtually non-existent.
Production EDM, for the most part, still uses standard toolroom EDM machines which include elaborate fixturing and/or bulky workpieces. For example, large C-framed machines capable of lifting several hundred pounds, are still being used with electrodes or electrode assemblies weighing only a few ounces. The result is an expensive, over-dimensioned machine tool capable of handling heavy electrodes and not necessarily able to respond correctly to small electrodes. In addition, there is a large waste of energy in running the relatively large, hydraulic servo system.
Another approach taken by EDM manufacturers is to build a custom EDM machine which is capable of performing only one job. Such a machine is not only costly, but delivery times are typically very long. Also, when the particular job for which the machine has been designed is completed, the machine is worth very little to the user. In any case, the original purchase can only be justified in economic terms if the machine is to be used for a relatively long run of jobs.
Consequently, there is a need for a flexible, yet accurate, modular EDM system which can be easily assembled to perform a given job and then can be taken apart and rearranged for the next job. This is to be contrasted with a standard, toolroom EDM machine which includes many features which are never used during production jobs. By buying a modular system, one need only buy components necessary for a particular production job.
Such a system should also be adaptable to accommodate such devices as rotating spindles having rod or wire electrode refeed, electrode holders, either single or multiple; slotting heads with automatic wear compensation; and automatic tool changers.